If these loose screws are holding your railings in place, there's two ways to fix it: Either go to the next larger screw from the usual # 8 to a # 10. (Avoid going to a # 12 if possible--looks ugly.) or, move the post slightly fore or aft and drill three new holes.

A bunch of us folks have dabbed that 3M 5200 on every screw we threaded into the aluminum, getting it all over the darn place, yet the corrosion continues. Gotta be a better way.

A bunch of "things" have to be attached to the aluminum of the boat with stainless screws, and that eventually causes corrosion at the stainless steel/ aluminum interface. Aluminum screws are worthless (very weak), and brass (gasp!) or carbon steel is out of the question. The trick is to (somehow) isolate the stainless screw from the aluminum.

Below are a few of the ideas—good and bad—on ways to reduce or avoid corrosion below the railing posts:

AN IDEA THAT DIDN’T WORK--USING RUBBER RIVET-NUTS:

Thinking of a way to attach the railing with SS screws without the screws touching the aluminum by using rubber nuts. Saw some the other day at my favorite hardware store, and these are NOT your ordinary wall hangers; these look like you could lift an engine with one, and they come in a variety of sizes. I'm thinking about a size to take a # 10 or # 12 SS sheet metal screw.
They would require drilling the stripped out (usually) hole in the deck to about 1/4 inch, pushing the "anchor" in the hole (with 3M 5200 on it) then zapping in a sheet metal screw through the railing foot. I'll buy a few and install them in some 1/8 aluminum plate, then try to rip a railing piece (from my old FB) out of the plate.

Well, I tried my experiment with the #8-32 rubber nuts and all 3 of the rubber nuts ripped out a bit too easily. They make larger rubber nuts, for 1/4 " bolts, but they require at least a 3/8 hole in the deck. So, back to the drawing board

HOW ABOUT ADDING A SLAB OF FIBERGLASS IN BETWEEN THE POST AND THE DECK?

I've been wondering if one couldn't do something similar to the drive savers for the railings. Seems like it wouldn't be all that hard to make a small disc type mold so one could make some "hockey pucks" out of epoxy with an aluminum or stainless disk molded in, but completely encapsulated in epoxy. If the disk had clearance holes that allow you to bolt the puck to the disk with flat head screws and another bolt pattern with threaded holes to fasten the stanchion base to the puck via the metal insert.

I might try bonding a sheet of fiberglass to the deck, then screwing it down. The railing posts would then be screwed to it with screws short enough to miss the aluminum. To be strong enough, the fiberglass pad would need to be about 1/2" thick and about 4 x 6 inches in size. (Ug-lee!) Some "artistic" shaping of the pad, and painting it deck color, would make it look a bit better.

I personally wouldn't want to rely on rails that are only screwed into fiberglass. There's a good reason that stanchions are usually mounted with backing plates on fiberglass boats; it's too easy to rip the mounting screws out of the fiberglass.

With the design I'm thinking about there is no direct metal contact between the stainless screws that would mount the "puck" to the deck, but since the screws go through the "backing plate" that would be molded into the puck you wouldn't be relying on fiberglass/epoxy to for the security of the mount - the fiberglass or epoxy would only be in compression not tensile. If the rail was stressed enough that the puck crushed, deformed, or whatever, the stainless screws would hit the metal plate and still prevent the puck/stanchion mount from being ripped off the deck until the stainless screws actually rip out of the deck. The stanchion base itself would be bolted through clearance holes in the puck into threaded holes in the stainless backing plate molded into the puck. I think you can put both the puck-to-deck mounting clearance holes and the stanchion-to-puck threaded mounting holes on the same bolt circle by just offsetting the angle of the two bolt patterns. As an added benefit by using the same bolt circle for both bolt patterns the puck could be round and just look like a thick stanchion base, which I think would be more cosmetically appealing. I'm thinking the puck would be around 3/8" thick; an 1/8" of epoxy with microfiber filler top and bottom, and a 1/8" thick stainless backing plate in the middle. The top layer might need to be a bit thicker to make sure that the flat head bolts used to mount the puck to the deck don't touch the metal backing plate in the counter sinks.

ON USING PLASTIC INSERTS (LIKE WALL HANGERS) IN THE DECK TO ELIMINATE THE ELECTRICAL CONNECTION:

I'm pretty sure the plastic inserts would work for some applications. I was wondering if some sort of thin plastic sheet materiel could be used to sleeve the existing hole. I've got bigger issues than the screws, like loose screws holding the railing in place.

ON FILLING THE HOLES LEFT BEHIND IN THE DECK AFTER THE SCREWS STRIPPED THEM OUT:

I’ve used Bond-O type materials for filling holes with backer but never did try to re-tap. Never had opportunity to use the Loctite re thread product but it may be worth a try on some scrap. Being that the aluminum is so thin it makes me wonder.

It’s very easy to blow holes right thru that thin-skinned aluminum unless you are a professional welder. Yesterday I tested 1/4"- 20 nylon flat head screw in a similar test as Jeff tested the rubber rivet. Had me rather excited momentarily. Side to side, it appeared to be doing well even with a good deal of pressure at the end of an 18" pc. of stanchion in the base. Then I gave it the shock treatment, though, and snapped all 3 flat head screws right where the threads meet the taper like tooth picks--certainly not the way to go.

ON ALUMINUM SOLDER TO FILL THE HOLES LEFT BEHIND IN THE DECK:

Welding supply counterman gave me one stick of "3 in 1" to test with. Forgot to note the manufacturer. Cost for a big package was only $18. It is Non-flux and melts at 400 degrees. I bought 2 lbs of "Extra Hands" heat sink clay for $15. Then I went to Home Depot. Got Benz-o-Matic mini torch and fuel for about $30, some clamps and some aluminum stock to test with.

The corrosion resistance of this "magic welding rod" will not equal that of the Marinette stuff. That means accelerated corrosion in that area, of either the parent metal, or the weld metal. Of course, if the weld is properly coated and protected it might be okay.

Tech stated not to use it below water line. A lot of the threads in this forum state one must use a # 5000 series type rod or corrosion will surely commence. I had good luck with soldering the scrap pieces together; held up well to twisting & turning metal fractured before welded joint. Now only if we could find some of these rods in the #5000 class of rod we be in business.

Also looking at aluminum brazing rods. The one I was looking at was the HTS 2000 supposed to be second generation and no longer does surface have to be completely clean. It has hardness of Rockwell 48 I wonder what are boats are. This one melts at around 750F.

HTS-2000 seemed like the best of the internet sites. As to corrosion they claim "Electrochemical corrosion is never a problem, because the electrode potential between HTS-2000 and aluminum is so slight". They called the rod given me at the welders supply, Alladin 3 in 1, a "first generation product". I still don't like the idea of buying $65 worth for a test.

I still don't recommend any soldering stuff below the waterline. (It's probably not a good idea above it either.) One thing to keep in mind with an aluminum hull, as I have sadly discovered, is that any "abnormality" in the hull tends to FOCUS the galvanic corrosion in that spot like a magnifying glass. That goes for missing bottom paint and 'different' aluminum areas.

ON USING SELF-PIERCING CLINCH NUTS:

They take a lot of work, but give you stanchion bases that as secure as a baby crib & come in 5052, 6061 and 1100 series as standard catalog items. Here's one source: http://www.multifastener.com, or http://www.fastenall.com/


ON USING ALUMINUM RIVETS:

I found some aluminum rivet nuts that go into a 1\4 inch hole then you pull them up (like a pop rivet, but far better).

I am going to give the aluminum rivets a try. The corrosion test samples (that sat out all winter) look fine, and without any paint or sealer. On the mounting system, I've not going to put a rubber gasket below the railing bases. I've given this a lot of thought. The rubber gasket traps moisture, causing crevasse corrosion in itself. I'm going to remove aft railing then reinstall it with the rivets. The rivets are a McMaster-Carr item # 94020A323. You can order them on-line at www.mcmaster.com. You'll also need some stainless 8-32 flathead bolts about an inch long, and some nylon flat washers. (Note: Later made up 1/8 thick x ½ “ OD washers from scrap plastic.)

I got my aluminum rivet nuts today and wanted to know if you used a special tool or mandrel?

You got 2 choices: Make up your own tool, or spring for the 10 buck tool sold by Harbor Freight.

Started the rivet/bow rail job. The deck was pre drilled before topside paint. Applied 4200 to insert and riveted in. Harbor freight has the threaded insert tool on sale for $9.99. Broke the insert tool after 6 rivets. Had to head back to store and replace it (with 2 more, just in case)

I too used the cheapo gun. Took about 5 attempts to sink 3 rivet-nuts. The problem is the mandrels. When you thread them out they tend to bend and strip out the inside of the rivet-nut. It helped to have a friend hold the gun tight and vertical. I do know a replacement mandrel was $40.plus at a fastening specialty store. To compensate for the little heads that ride up, I used a slightly thicker gasket material and cut out where the rivet-nuts would be. After they were in I smeared some 5200 and also used some when I ran the 10/24 screws in. I used that size because there was a mandrel that size in the kit. The 10/32 would offer more thread bite.

There can be as much as 0.2V difference between various marine aluminum alloys so if water gets into the rivet-nut/deck joint there is still some potential for corrosion, although the majority of the corrosion would be between the rivet-nut and stainless screw.

Are your threaded rivet inserts round or the hexagonal shape? I'm wondering if one would aid more in not spinning out. Trying to figure out which way to go. (They are round with serrations in the 'upset' portion. )

Will they cause the hull aluminum to corrode? Time will tell, but the rivet itself—which should corrode before the deck material, is easily drilled out and replaced.

The rivets are made of 5056 aluminum; good, but not as good as our 5086 so they will corrode first, and that's the whole idea. One can drill the head off one of these rivets in seconds and install a new one. Strength-wise, unless the rivet is heavily corroded, it will hold as good as the screws. At least 50 % of the screws on my boat were loose (to the point of falling out) when I got it. After a year of use, a bunch of them had loosened up once again. I can't see these rivets doing that.

I use aluminum rivets and bolts to hold my railings on and they are are SOLID! The deck would buckle before the rivets pull out or the #8-32 bolts snap off, which is a vast improvement over those whimpy screws--and I have yet to find a bolt getting loose on me. The screws constantly needed retightening.

ON THE RAILING POSTS LOOSENING UP OVER TIME WITH ALUMINUM RIVET-NUTS:

So far (three years) they still hold very strongly, much more than the screws.

SOME GENERAL IDEAS AND COMMENTS ON THE SUBJECT:

Does anyone know a good source for those gaskets for stanchions
etc that go between deck and bases? (You’ll have to cut your own from sheet material.)

I think the rivet-nut is a potential fix for loose railings, but there is still the lack of isolation with the hull/ rivet-nut/ and bolt. Your idea with the plastic anchors sounds interesting. Has anyone heard of aluminum stanchion bases? You could go aluminum for the whole safety rail or just isolate the stanchion in a boot that fits in the base. Every Marinette I see has corrosion around the stanchion base.

I'm toying with the idea of an aluminum stanchion base. It would be mounted to the deck with an aluminum rivet-nut and the screw would be a #10 aluminum screw. The railing could still be stainless if I isolated between the stanchion and the base with a boot of some sort. Ideally, I think I would go all aluminum: rail, stanchion, base, rivet-nut, and screw. I need to do some strength tests. I've got to think that a bolted aluminum fastener is going to have more holding power than a threaded stainless screw into 1/8th of aluminum deck. How many threads actually bite? With 3 holes per base I'll take the weaker aluminum screws and a rivet-nut. Anyone know of a supplier of aluminum stanchion bases? I hope to have this beat before the end of the season.

Using an aluminum stanchion base won't cure the problem, for those pesky stainless screws are still in use, doing their evil work. We need to totally isolate the SS parts--railings and screws--from the aluminum hull.

What I was trying to accomplish is removing any dissimilar metals and fabricate a railing entirely out of aluminum. It’s more cost effective and more readily available than stainless. But how do you fasten an aluminum railing to deck? I can't get to the backside of the deck so that leaves me with the isolated stainless screw approach.

I think there are two classes of stainless screws. One is regular stainless, the other is referred to as passivated, which means it has control of the impurities in the screw, so it won't cause galvanic corrosion. I will look for the source of this special stainless screw. Passivation is just a thin coating on the stainless that prevents the steel from corroding. It does little if anything to reduce galvanic action. There are two general types of stainless steels, austenitic (300 series) and martensitic (4000 series and others). The latter is stronger, but is much more noble than aluminum. Austenitic stainless is less noble than martensitic yet slightly more noble than aluminum, and is acceptable for use in direct contact with aluminum, but there will always be some corrosion in the aluminum. There is no perfect solution to this problem of fastening the stanchions to the deck.

USING A THICK PLASTIC WASHER AND LONGER SCREWS TO HOLD THE RAILINGS IN PLACE:

I put thick nylon washers under the aft railing stanchions. I think I used 1/8" thick. Used longer screws w/ more thread. Railing is tight as a monkey's fist and you can shoot the hose under each one to keep them clean. Ill do the entire bow rail in the spring. The height was an issue to me before I used the washers. I too thought it would be sloppy but they are solid. I think the harder nylon keeps the flange tighter and more rigid then the soft rubber pads initially used. Sometimes it’s a pain screwing into even in an existing hole. It’s easy to torque off screw heads. If it gets hard to turn, back the screw out and try again. I think putting a little sealant like 3M 5200 on the threads before hand. I think this will prevent water from working down between the threads and aluminum.

Are you sure you want to use an adhesive sealant on screws that some day may have to be removed? The 3 M 5200 is pretty much a permanent type sealant. Most marine catalogs have just as good sealants without adhesives especially for above the waterline (like 3M 4200). Those screws will be hard enough to remove after a few years without the use of an adhesive.

ON THE BASIC PROBLEM IN ITSELF:

As I understand it, the corrosion can only occur if there is water (or some other electrolyte) at the interface between the two metals. If that's true then you should be able to minimize the deck corrosion issue by bonding a cast epoxy pad (or fiberglass) to the deck. A properly bonded pad would prevent water from coming in anywhere. Water entry around the screw shank can be minimized by putting a countersink in the pad where the screw enters and providing an O-ring-like seal around the screw.

Water's going to get in. It runs down the upright rails and into the feet and if you're not careful, gets trapped there for a long time. I replaced the rubber pads with thicker ones made of neoprene and cut wider, deeper grooves on the bottom of the rail feet. The grooves gave the water an easy way to get out. The ones cast into the feet were just too shallow and were easily blocked. The bad news is that that famous Marinette rail stanchion paint blistering is beginning to reoccur. Good news is that it has taken 5 years and at a slower rate. There's an insulating goop you can use to coat the screws but I think most of that is scraped off as the screw bites into the aluminum. Don't know what it's called, but Loctite is one of the mfrs.

HOW ABOUT ALL-ALUMINUM RAILINGS?

All aluminum railings would have to be thick and heavy to approach the strength of stainless. If screwed on, the screws would need to be very heavy like the ones on the cleats. (Note: They would be welded on!) I suppose it could be done, although it would be bulky.

HOW ABOUT ISOLATION OF EACH SCREW FROM BELOW?

By pulling out the interior paneling, would it be possible to get to the underside of each stanchion? (Maybe…) If so, one could bolt the railing on using, a plastic sleeve to isolate each fastener, that would prevent contact with the aluminum.

The classic Marinette railing corrosion problem will happen long as SS screws connecting SS railings to an aluminum hull. Some feel that the gasket material doesn't matter, and that corrosion would be much worse without a gasket. (Others feel the gasket itself IS the problem.) Like most of you, I too am frustrated with this. My personal "icebreaker" was repainted about 5 years ago and the corrosion is back already. I've been studying the use aluminum pull up rivets that have an internal thread for an #8-24 bolt. The trick here--if it will work--will be to let the aluminum rivet (that's easily replaced) do the corroding instead of the hull around the SS screws.

ON THE EFFECTS OF CARBON IN THE RUBBER CAUSING CORROSION:

The rubber strips that were used for isolators in several places on my boat certainly appear to be causing corrosion. When I replaced my fuel tank the only area that had corrosion was where an aluminum bar was pressed against it with only a rubber strip between them. The rubber seems to have plastic (self-adhesive) film on it. Where this was disrupted there was significant corrosion. Stainless will also react with carbon-based rubber.

The culprit, if you read about aluminum fuel tank problems, is corrosion caused by trapped water. Smearing a film of 3M 5200 under the rubber pad used to support the tank is recommended to cure the corrosion there.

I'm a bit skeptical about the carbon in the rubber being part of a galvanic cell.
I’m thinking that there has to be an electric current to cause all the bad stuff to occur, but I’ve been wrong before.

When I re-powered I had a tiny hole corroded through one of my exhaust flanges. This is a 4" diameter piece of aluminum 3/16 or 7/32 thick. The corrosion was from the exhaust hose side all the way through. The only two materials in contact with each other were the exhaust hose and the aluminum. There were some other pits and significant corrosion, and all of them were under the rubber. There was no corrosion where the seawater & exhaust passed through the tube. True this may be due to trapped water or exhaust byproducts, but wherever I have had rubber and aluminum in contact with seawater I have had pitting. (Note: This is also true below the aluminum step plates screwed to the aft deck.) If the parts are fully bedded and sealed in with 3M 5200, you won't get any corrosion until you get the tiniest penetration of water.

Don't wait too long to drill out the deck or you may find you only have about 1/16th wall thickness to work with after you grind out the corrosion. (Rust never sleeps). The previous owner had the boat repainted--with rotten prep job--so they may have already sanded some aluminum away.

IS RADIO FREQUENCY THE PROBLEM?

The railings act like a big antenna. They are receiving RF energy 24 hours a day. That energy then seeks ground thru the fittings and creates the current that damages the aluminum. Isolating the screws will minimize the corrosion problem, but it may create another by not giving the hot "antenna" a path to ground and building voltage. You may need to run a dedicated wire or two from the railing to the common grounding point to avoid becoming a human ground when you grab the rail (the resulting spark could be enough to cause ignition). I'm sure you could think of a clever, unobtrusive way to accomplish this.

Wasn't thinking about the railings being an RF antenna, though they do tend to look like the radio antenna setup the Pennsylvania RR used on their early diesels. Maybe the wire could run from one of the attachment screws under the deck. That way there wouldn't need to be any more holes drilled

That'd work, long as the wire went to the water without touching the hull.

Perhaps it would be best to run several insulated ground wires from the screws to the common grounding point because you need it when running the boat to prevent the railing from acting as a big capacitor. Not convinced the railing would become dangerous if it was isolated for the railings on f-glass boats are isolated all the time. I've yet to hear of someone getting zapped on them, but it's still an interesting theory. I can remember fishing with some friends on a fiberglass boat with a storm going by. My friends had a graphite rod and I had a fiberglass rod. I was ok the other guy kept dropping his poles from getting zapped. His graphite rods was very good lightning rods


TWO POSSIBLE WAYS TO STOP THE DECK BUBBLES:

1) Keep the water out. If the there is no water where the stainless screws go into the aluminum you can't have corrosion. The galvanic corrosion occurs because there are dissimilar metals immersed in an electrolyte, in this case water. Salt water is an even better electrolyte, which makes the problem worse.
2) Isolate the dissimilar metals with an insulating material. Marinettes use this approach for the prop shaft coupling.

ON USING ALUMINUM SCREWS INSTEAD OF SS:

Don't know if I'd trust them. What's your experience with them? (They are fairly weak fasteners. A # 12 would be as weak as a # 8 SS screw.)

ON USING AN ALUMINUM CHANNEL TO HOLD THE RAILING POSTS IN PLACE:

Here is an answer, but it would require a bit of welding. The idea is this: weld a shallow, slotted channel of marine aluminum onto the deck at each stanchion. The channel would need to be about three inches long, a half inch high, and wide enough to enclose the mount. Close off the aft end so that its open forward only. Slide the railing mount into the channel and attach the railing. Seal the opening with silicone to keep water out. If there is corrosion, it will be inside the channel and not visible. An insulating sleeve around the mount would eliminate corrosion, but I'd rather have the railing grounded. The system would be safe as the channel won't break off or come loose like screws.

I assume the "U" is downward, with the railing screwed to it instead of the deck? This way the channel takes the corrosion hit instead of the deck. Why seal the aft end shut? A negative would be an even greater chance of getting a "Marinette toe" while walking the deck.

Are you thinking of using screws into the slotted channel or just sliding it in?
I see a couple problems if the plan is to just slide it in. First it will be difficult to keep the area sealed with a pure slide in mount as it will necessarily have to be a very slightly loose fit to allow it to slide in, which will allow a bit of motion after its in. The more things move the faster a caulk seal will be broken. Once the seal is broken the channel would trap water, which would accelerate corrosion. On the plus side the corrosion should be inside the channel where it isn't a cosmetic issue. Also I would think the potential for buzzing noises from vibration would be quite good, which is something that drives me nuts. One last thing I see with this approach is that you would only want to do it if you already need to repaint the deck. Since my deck and cabin was just repainted with Imron I don't think I could talk myself into welding anything to the deck.

The channel would be similar to your gutters with the only difference would be the size. No screws involved the foot of the stanchion would slide in the
slot of the channel. Trying to find a marine AL that had a slot that size may be another story. I would think they’d (feet) have to fit rather snugly so rail wouldn’t rattle underway. The caulking I’m not sure about? If someone was repainting their boat this idea may be worth some exploring.

But... your interpretation may be on to something. Welding an upside down U-bracket to the deck would limit the corrosion to the U-bracket, which would be relatively easy to repaint as corrosion starts appearing. Another plus is that you could use bolts with nuts to mount the stanchion base, so no more worries about needing to go with oversize screws as the holes in the deck get enlarged over the years. Another possibility would be to weld a 1/2" or so thick aluminum disc the same diameter as the stanchion mounts to the deck. This "boss" would be threaded with the same bolt pattern as the stanchion base. It would probably look a bit better than the U-bracket and the corrosion that occurs would be limited to the top of the boss, which again is easier to repaint than the deck. There's got to be an answer to this dilemma!

No screws involved. A refinement would be to taper the height so that the channel becomes shallower the deeper a mount is inserted. That would eliminate rattles from vibration by keeping it snug. You could then eliminate the sealed aft end and just apply silicone to keep the moisture out

I would think it would be better to allow water (under the railing posts) to flow freely. Water always wins that way when you wash the deck and will flush out the corrosive dirt that collects in the area.

ON THOSE OVER-THE-WINTER CORROSION TESTS:

I’m also wondered if the 4 to 6 month corrosion test is long enough? (Agreed!) I noticed a couple of bubbles after 15 years, & someone else said after working on their rail corrosion it’s lasted 5 years or more. If after repairing these two spots I get 5 yrs. service out it.

ON WAYS TO REMOVE THE RAILINGS:

I pulled off about half the railing last spring on my boat when painting it. It was a slow process since half the screws striped out. Ended up drilling off the heads of the stripped ones, removing the rail and then using vice grips to unscrew then the rest of the way. I did not even get all the rails off because it was taking so long. It was dirty under there. Maybe I should have kept going.

Do yourself a favor & get an impact driver. It does wonders on take out Phillip head screws. Just be sure to practice a while with that thing before you try removing the screws. It does take a bit of practice. The trick is to push really hard on it while twisting it CCW before whacking it with a hammer.

I use the impact driver a little differently: I insert Phillips head driver in the slot, give it a gentle tap to set it. Apply a CCW twist to activate the tool and give it the second hit, which in most cases is sufficient. I then use a conventional or electric drill to back it out. Using this method, only 2 screws out of 70 + gave me a hassle. and they where the ones at the bow
under the pulpit. I do agree, practice, caution & common sense must be applied, also safety glasses. Whichever method works best for the person using the tool is the proper one.

ON LEAVING THE RUBBER GASKETS OUT BELOW THE RAILING POSTS:

It makes sense. I do think that with people pulling and pushing on the railing like they do, that spacer does cushion and tighten up that wobble a little. You might find the railings working themselves out a little sooner without it.
Even when a stainless cleat is isolated from the aluminum hull by a spacer (rubber of plastic) the stainless screws that attach it still provide an electrical path. That's why we always end up with the dreaded "Marinette Railing Mess" after a while, no matter how the railings are attached, for dissimilar metals don't like each other. SS screws are not a friend of an aluminum hull.
Consider this: If there were no SS screws holding the railings to the hull, would you still have corrosion between a spacer and the hull? You would not, for there would not be any contact between metals.

From what I've observed, if the railings are well screwed down (and I too had to go with larger screws in places), the deck flexes when some squid yanks on the top of the railing (instead of at the base). But I have to disagree with you: I do not think that's why the corrosion occurs. It's those ~#@%#$ rubber gaskets! Even railing posts in the rear--that are not under much stress--had ugly corrosion going on in the paint under the rubber gaskets. One further piece of evidence: The paint below the footplate was also dirty and ugly, but there was minimal corrosion. Toss out all of those rubber gaskets!

ON WHY THE RAILING POSTS HAVE A CORROSION PROBLEM WHERE OTHER SIMILAR CONNECTIONS DO NOT:

I know we have been around this issue several times but I realized something the other day and want to see what others think about it. Most of us seem to be battling corrosion around the bow railing flanges and the deck. Some have tried thicker spacers some have attempted aluminum screws and rivets etc.
My question is why is there no corrosion on the flanges that meet the bottom of the aluminum stern railing and no corrosion on the flanges that hold the hand railings to the cabin tops and sides? Perhaps some boats have corrosion in these places but I think for the most part the deck area is the worst. After studying these locations for sometime ( I had plenty of time for this while I was fishing and not catching anything) I realized the only variable was water. The cabin railing flanges are flush, there is no cavity to retain water and gravity takes care of any water that may work its way into the flanges directly under the stern railing. Four yeas ago I drilled the weep holes bigger in all the railing flanges to encourage drainage. I have had zero blisters until I noticed a tiny bit of bubbling today by one flange. I used high pressure water to blow into the weep hole and found lots of mud & dirt squirting out.
My conclusion is that it does not matter how the railing is fastened to the deck or how thick the rubber spacer is but how well dirt, mud and water can be flushed from each flange allowing the surrounding to be clean and dry.
The railing mounts on the stern and cabin tops and sides are cast aluminum and fastened with aluminum screws, so there is no galvanic corrosion where the railings are also aluminum. The stanchions supporting the stern rail are stainless steel. My cast aluminum stern rail mounts have completely rotted away after 36 years of service, and I am currently replacing them with stainless mounts. Interestingly enough, the aluminum screws are solid, more so than some of the stainless screws I have replaced. I never would have thought aluminum screws could be reused, but they are doing just that. When railings were removed to paint deck this winter, I noticed most of the corrosion was around bow rail compared to almost none around aft rail. My bow rail was significantly looser than the aft railing. Could the action of pulling and pushing allow water on the deck to work its way in and do its damage? Not in the budget but how about a welded aluminum rail? 1 piece all around bow and then tied into the aft rail... dare to dream!

The moist dirt that gets trapped under the railing posts sets up a corrosion cell that eats up the deck. As to why railings on the cabin roof do not. I suspect these railing don't get wiggled, which might not allow dirt to enter. With screws holding the railings in place...